Makindo Medical Notes.com

Related Subjects: |ECG-QT interval |Brugada Syndrome |Long QT syndrome (LQTS) Acquired |Long QT syndrome (LQTS) Congenital |Torsade de Pointes |Ventricular Fibrillation |Ventricular Tachycardia |Resuscitation - Adult Tachycardia Algorithm |Automatic Implantable Cardioverter Defibrillator (AICD)

Consider the diagnosis in any patient with polymorphic VT or syncope and focus on drugs and electrolytes as causes. Torsade may be due to inhibition of the HERG type potassium channel. QT prolongation is considered when the QTc interval is greater than 440 ms, but arrhythmias are most often associated with values of 500 ms or more.

About

• An updated list of causative drugs can be found at www.qtdrugs.org
• Over 50 known drugs lengthen the QT and this is increasing
• Arrhythmias are associated with prolongation of the QT interval
• Risk of VT, syncope, cardiac arrest, sudden death.

Torsades

Aetiology

• QT is a period of Ventricular repolarisation and if prolonged acts as a substrate for arrhythmias
• Can lead to polymorphic VT (Torsades de pointes) and VF
• May be a subclinical genetic predisposition either in cardiac tissue or in metabolism of particular drugs

Causes

• Medications: Antiarrhythmics (e.g., sotalol, amiodarone), antibiotics (e.g., macrolides, fluoroquinolones), antipsychotics (e.g., haloperidol, ziprasidone), and others can prolong the QT interval.
• Electrolyte Imbalances: Hypokalemia, hypomagnesemia, and hypocalcemia are common contributors.
• Bradycardia: Especially in combination with QT-prolonging drugs.
• Structural Heart Disease: Conditions such as myocardial infarction or heart failure can predispose to QT prolongation.

ECG appearance

Drugs that may lengthen QTs (not comprehensive so check BNF)

• Look for Antihistamines, Antibiotics, antipsychotics and Cisapride
• Terfenadine, Astemizole, IV Erythromycin, Clarithromycin, Phenytoin, Lithium,
• Chlorpheniramine, IV Prochlorperazine, Tricyclic antidepressants, Phenothiazine, Halofantrine
• Erythromycin, Phenothiazine, Sotalol, Amiodarone, Class 1a antiarrhythmic drugs, Tricyclic antidepressants, Chloroquine
• Miscellaneous - Acute MI, Starvation, Bradycardia, Myocarditis and HCM, Hypothermia, RV dysplasia, Methadone

Clinical

• Commoner in women. Take a drug history
• Asymptomatic, syncope, cardiac arrest, sudden death, seizures
• Symptoms may be with exertion and also at rest
• Always ask about a Family history of SCD important

Investigations

• U&E, Magnesium, Calcium
• ECG: Long QTc: Normally, the QTc interval varies from 0.35 to 0.46 seconds (350-460 milliseconds). About 95% of people have a value between 0.38 and 0.44 seconds, which is the range doctors generally consider normal. A QTc interval >450 ms in men and >460 ms in women is considered prolonged.
• ECG: VF or Torsade de pointes (polymorphic VT)

Management

• Torsades: IV Mg is the agent of choice for immediate treatment of torsade de pointes irrespective of the serum magnesium level. 2g bolus of magnesium sulfate is followed by intravenous infusion of magnesium
• Identify and manage causes when QT prolonged
• Monitor those with a QT > 500 msec and get a cardiology consult
• Do not use Amiodarone or other antiarrhythmics if possible - can make it worse
• Stop any drugs which may be to blame.
• Check and Correct K and Mg
• IV Magnesium can be used. IV Lidocaine can also be used (1)
• Overdrive pacing or just simple pacing if patient's VT is bradycardia dependent and prevents long RR intervals which can predispose to arrhythmias. Isoprenaline has also been used to speed up the heart rate.
• Defibrillation is the treatment of all unstable tachyarrhythmias

References

• Acquired, Drug-Induced Long QT Syndrome
• Mechanisms, Risk Factors, and Management of Acquired Long QT Syndrome: A Comprehensive Review